With a stretched upper deck the entire length of the fuselage, the B747XS would be able to accomodate more passengers and thus lower seat/mile cost. Also, with the stretched fuselage, there will be more room to accomodate cargo/baggage.

Structurally probably harder to achieve than the current configuration: The flat areas on the side of the upper deck are structurally the most problematic points of the airframe (because of the pressure acting on a flat surface instead of curved one), extending that to full length might be impossible. Also, the aerodynamic efficiency is assumed to be optimised at the current upper deck length (i.e. ending just before the wing) which is why I don't think Boeing would ever extend the upper deck past the onset of the wing.

No, Airbus didn't stretch the narrowbody upper deck to the full length. They just designed (will design) a new airframe. The difference is that A380 is (of course ) intended to have such a layout while B747 is probably not. And, even if yes, the stretched upper deck will be tooooo long to have a single aisle... and to make it wider would mean to build a new airframe - exactly as Airbus did.